The present invention relates to spatial light modulating devices in which a refractive index distribution of an electro-optic crystal is changed in accordance with an input optical or electron image and the stored image is read out through modulation of readout light in accordance with the refractive index distribution.
FIG. 1 shows a "PROM" (Pockels Readout Optical Memory) structure which comprises a BSO (Bi.sub.12 SiO.sub.20) single crystal wafer 101. The BSO crystal exhibits both of a photoconductive effect and an electro-optic effect. In FIG. 1, both surfaces of the BSO wafer 101 are coated with transparent conductive films 103, 104 for voltage application through an insulator (parylene) 102. When, under the application of appropriate voltage between the transparent conductive films 103, 104 an image carried by ultraviolet light or blue light is formed on one crystal surface, electron-hole pairs (electric charges) are generated at the illuminated areas and the electric charges are trapped in a boundary between the insulator 102 and the crystal 101 by means of a drift electric field caused by the applied voltage. An internal electric field distribution, which is formed by the trapped electric charges, causes a refractive index distribution of the crystal 101 through the electro-optic effect, where the crystal 101 has a double refraction property. In order to avoid erasing the input image, infrared light or red light is made incident on the other surface of the crystal 101 as a readout light. The readout light is modulated in accordance with the refraction index distribution when it is reflected by the crystal surface. This means readout of the input optical image.
FIG. 2 shows a "Photo-Titus" structure which utilizes a DKDP (Deuterated Potassum Dihydrogen Phosphate, KD.sub.2 PO.sub.4) crystal 111. A photoconductive layer (amorphous Se) 113 is formed on a front surface of the DKDP crystal 111 through a dielectric mirror 112. Moreover, both surfaces are coated with transparent conductive films 114, 115. When an input image is formed on the photconductive layer 113, the input image is transformed into an electric charge image and stored in a boundary between the dielectric mirror 112 and the photoconductive layer 113. A refractive index distribution of the DKDP crystal 111 is changed by an internal electric field distribution which has been formed in accordance with the electric charge image, where the DKDP crystal 111 has a double refraction property. The refractive index distribution can be read out by making readout light incident on a back surface of the DKDP crystal 111, where the readout light is modulated when it is reflected. This means readout of the input optical image. In another structure called "Titus", image information is inputted by scanning the front surface of the DKDP crystal 111 with an electron beam which is emitted from an electron gun, instead of incorporating the photoconductive layer 113.
FIG. 3 shows the constitution of a microchannel spatial light modulator (MSLM) 3. A photocathode 5 and a light modulating plate 9 consisting of an electro-optic crystal (LiNbO.sub.3) are equipped on respective end surfaces of a vacuum-sealed cylindrical tube 4. Between the photocathode 5 and the light modulating plate are disposed a focusing electrode 6 which focuses electron beams carrying an electron image which has been emitted from the photocathode 5 on a surface 92 of the light modulating plate 9, and a trapping electrode 8 which traps secondary electrons which are backwardly emitted from the surface 92 of the light modulating plate 9. An electron amplifier 7 such as a microchannel plate (MCP) is incorporated when it is required. The MCP amplifies an input electron image through a secondary electron amplifcation phenomenon.
With the above-described constitution, prescribed voltages are applied to the electrodes from respective voltage supplies A, B, C and D. When an optical image is projected on the photocathode 5, electrons are emitted therefrom corresponding to the projected optical image. The emitted electrons are then focused, amplified, accelerated and finally made incident on the surface 92 of the light modulating plate 9. An electric charge image is formed through direct charging by the incident electron beams or through secondary electron emission. The light modulating plate 9 is made by polishing a LiNbO.sub.3 crystal extremely precisely to form a plate of uniform thickness. An electrode 91 is closely contacted to the other surface of the light modulating plate 9. A refractive index distribution is changed by an electric field distribution which is formed by the electric charge image on the surface 92 and the voltage applied to the electrode 91.
Light beams emitted from a point light source 13 pass through a focusing lens 12, a monochrome filter 11 and a half-mirror 10 and are normally made incident on the light modulating plate 9. The incident light beams are modulated in accordance with the refractive index distribution, so that light beams reflected from the light modulating plate 9 produces, after reflected by the half mirror 10, an output optical image corresponding to the input optical image on a surface 14.
In this manner, the microchannel spatial light modulator 3 can amplify the input optical image and, if the point light source 13 is a laser light source, can convert an optical image of incoherent light into an optical image of coherent light.
Instead of using the photocathode 5 and the MCP 7, the electric charge image may be formed on the light modulating plate 9 by scanning the light modulating plate 9 with an electron beam emitted from an electron gun.
In the meantime, in such spatial light modulating devices as described above which utilize an electro-optical crystal, the spatial resolution of the device greatly depends on the thicknes of the electro-optic crystal. This will be explained hereinafter in the case of the microchannel spatial light modulator 3 shown in FIG. 3.
As shown in FIG. 4A, if the light modulating plate 9 is relatively thick, an electric field caused by an electric charge at a minute point P on the surface 92 widely extends around the point P as indicated by a symbol .delta..sub.1. On the other hand, as shown in FIG. 4B, if the light modulating plate 9 is relatively thin, an electic field caused by an electric charge at a minute point P narrowly extends around the point P as indicated by a symbol .delta..sub.2. For example, while the spatial resolution with the crystal of 300 .mu.m in thickness is about 2 lp (line pairs)/mm, it is improved up to about 10 lp/mm with the crystal of 50 .mu.m in thickness.
The electro-optic 9 can be polished to the thickness of approximately 300 to 500 .mu.m with satisfactory parallelization and flatness. However, a usual polishing method cannot realize the crystal thickness less than the above value without curving the crystal. A curved crystal is not suitable for actual use.
In order to overcome this problem, the technique disclosed in Japanese Patent Application Unexamined Publication No. 166916/1984 was proposed. With this technique, a crystal wafer of 50 .mu.m in thickness can be produced with satisfactory parallelization and flatness by preliminarily bonding a crystal to a thick substrate with an adhesive and then polishing the crystal. Though this technique realizes a microchannel spatial light modulator having good resolution, another problem of increase in a voltage applied to the electrode has arisen. This problem is caused by the fact that part of the electric field caused by the electric charge image is developed in the adhesive layer, whereas the electric field should be developed only in the crystal.
In general, output light intensity from the electro-optic crystal periodically varies with a voltage applied to the crystal. A "half-wave voltage" which corresponds to the difference between a maximum and minimum of the output light intensity is also called the "operational voltage". For example, in the case that the operational volage is 2 kV, crystal thickness is 50 .mu.m, and thickness of the adhesive layer is 3 .mu.m, a voltage to be applied to the electrode 91 of the device 3 amounts to more than 3.5 kV. Such a high voltage causes a problem of voltage breakdown of the device 3.